Patent GB-760118-A (GB)
Title: Automatic calibrator employing electronic and photographic methods for the calibration of heterodyne wavemeters
Inventor: WILL COLIN
IPC: G01R23/00
Filed: 1953-07-31
Abstract: 760,118. Frequency measurement. BRADLEY, Ltd., G. & E. July 19, 1954 [July 31, 1953], No. 21372/53. Class 37. Apparatus for calibrating heterodyne wavemeters and like signal generating instruments generates a series of harmonically related calibration frequencies of known accuracy which are compared in a beat frequency circuit with the wavemeter output which is varied over its working range by a motor through a mechanical drive which simultaneously operates a mechanical counter indicating the current wavemeter frequency; the heterodyne output of the heat frequency circuit being arranged on passing through each successive heat frequency to operate a device for automatically recording the instantaneous reading of the counter. In Fig. 1 a standard frequency source A is at e.g. 20 kc/s. connected through a 1/ 10  frequency divider DV to a squaring pulse generator circuit CA generating square pulses at a recurrence frequency of e.g. 2 kc/s. which are applied to mixer circuit E together with the amplified output of a heterodyne wavemeter WM having a working range of e.g. 100-300 kc/s. The tuning control of the latter is mechanically varied over its range by motor F through gearing Z also driving a multi-digit counter N and a 10-way commutator L in synchronism therewith. The mixer output is connected through a low-pass amplifier G controlling a monostable trigger circuit TC so that .since the mixer input pulses contain multiple harmonics, the mixer output passes through zero beat every 2 kc/s. of the wavemeter range to impulse the trigger circuit which, through commutator L in its conducting condition triggers a thyratron KA firing a flash tube O instantaneously illuminating the counter to record the appropriate wavemeter frequency on the film of camera P; the commutator ensuring that the least significant digital index of the counter is correctly registered with its viewing aperture at the instant of illumination. A further thyratron KR is triggered on the termination of the pulse of trigger circuit TC to energize relay KR which operates the solenoid film traverse device Q in preparation for the recording of the next calibration point. In a modification (Fig. 2, not shown), the wavemeter tuning control is varied in constant positional increments by a step-by-step motor energized from a power source over the normally closed contacts of a control relay. On occurrence of each zero beat a thyratron is fired by a trigger circuit to energize a printing counter driven in synchronism with the wavemeter control to record the instantaneous value of frequency on a paper tape which is then automatically advanced one step. Simultaneously with the printing, a second trigger circuit energizes the control relay to interrupt the motor supply and hold the counter stationary during recording, after which the trigger circuits and the relay are released in readiness for the recording of the next calibration point. Fig. 3 shows a further modification wherein the source A produces a pulse train recurring at the standard frequency, which is squared and applied to a single shot multivibrator B producing for each input pulse an output pulse whose time duration is dependent on the upper frequency limit of the wavemeter. From this pulse train a variable frequency R.F. amplifier D whose tuning control is mechanically varied by motor F in synchronism with the tuning control of the wavemeter WM under calibration, sequentially selects a series of calibration frequencies f1 harmonically related to the standard source frequency, which are mixed with the wavemeter signal f2 in heterodyne circuit E, whose output is passed through low-pass filters to an amplifier G feeding a detector H. At inequality of f1 and f2 the rectified output applied to differential amplifier I energizes a relay J connected between the cathodes of the valves thereof to open its contacts and de-energize relay K, which is energized by the closure of such contacts at zero beat and holds over contacts K<SP>1</SP> until a ten-way commutator driven through gearing Z in synchronism with multi-digit counter N is in a position at which the least significant digital index of the counter is correctly registered with its viewing aperture. The release winding of relay K is then pulsed to open the holding contacts K<SP>1</SP> and the relay is de-energized to close contacts K<SP>2</SP>, K3, which respectively energize a flash tube O illuminating counter N for recording by camera P, and energize solenoid film traverse device Q in preparation for recording the next calibration point. The continuous calibration record may be cut into sections for binding into a booklet accompanying the wavemeter.